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Infectious Diseases

09 July 2020

Airborne spread of the coronavirus: Some scientists say yes, but WHO remains unmoved

An open letter to the WHO requests that they start addressing mounting evidence that Covid-19 is spread through the air.

  • Almost 240 scientists from 32 countries want the WHO to recognise the airborne spread of coronavirus
  • They claim that viral microdroplets are small enough to float in the air
  • WHO says that airborne spread is possible, but that the evidence not 'definitive'

The science community again seems to be at loggerheads with the World Health Organisation (WHO) – this time over downplaying the airborne spread of the coronavirus.

In a letter called 'It Is Time to Address Airborne Transmission of Covid-19', published in Clinical Infectious Diseases on Monday, over 230 scientists called out the WHO's overemphasis on infection through contact surfaces and not regarding airborne transmissions on the same level.

Potential for airborne spread

The two lead authors are Lidia Morowska, a professor of atmospheric sciences from Queensland University of Technology, and Donald K Milton, a professor of environmental health at the University of Maryland. 

"We appeal to the medical community and to the relevant national and international bodies to recognise the potential for airborne spread of Covid-19," urges the letter.

"There is significant potential for inhalation exposure to viruses in microscopic respiratory droplets (microdroplets) at short to medium distances (up to several metres, or room scale), and we are advocating the use of preventive measures to mitigate this route of airborne transmission."

WHO addressed some of their concerns in their weekly press briefing on Tuesday, with WHO's technical lead Benedetta Allegranzi saying that while airborne transmissions can't be ruled out, the body of scientific evidence is still growing and not definitive enough.

Droplet size the main concern

According to the authors and other signatories from 32 countries, the microdroplets are small enough to linger in the air after exhalation from an infected person.

They cited a study from China where members of three different families tested positive after visiting a restaurant. However, no one else who was in the restaurant tested positive, including staff. There was no physical contact between the families, but they all were sitting underneath the same air conditioner. 

"Other viruses have been shown to survive equally well, if not better, in aerosols compared to droplets on a surface," according to the letter.

Currently, the WHO focuses on sanitising surfaces, hands, face masks and physical distancing. 

The letter adds: "Handwashing and physical distancing are appropriate, but in our view, insufficient to provide protection from virus-carrying respiratory microdroplets released into the air by infected people."

This is especially relevant at indoor mass gatherings with bad air circulation where super spreader events have occurred despite sanitising protocols. 

Extra measures

The letter also lays out some measures that should be implemented to prevent airborne transmission:

  • Provide sufficient and effective ventilation (supply clean outdoor air, minimise recirculating air) particularly in public buildings, workplace environments, schools, hospitals, and care homes for the aged.
  • Supplement general ventilation with airborne infection controls such as local exhaust, high-efficiency air filtration, and germicidal ultraviolet lights.
  • Avoid overcrowding, particularly in public transport and public buildings.

"We are concerned that the lack of recognition of the risk of airborne transmission of Covid-19 and the lack of clear recommendations on the control measures against the airborne virus will have significant consequences: people may think that they are fully protected by adhering to the current recommendations, but in fact, additional airborne interventions are needed for further reduction of infection risk."

The authors believe this is especially important as most countries, including South Africa, are coming out of lockdowns and public exposure is increasing.

WHO's response

While the WHO didn't indicate any change in stance because of the letter, Allegranzi indicated that more evidence needs to be gathered and interpreted.

She also stated that they already recommend enough measures that take into consideration the possibility of airborne transmissions.

"We do recommend as much as possible avoiding closed settings and crowded situations. We recommend appropriate optimal ventilation of indoor environments and physical distancing, as you know.

"And when this is not possible in areas with community transmission of the virus, we recommend the use of face masks – in particular fabric and no medical masks for the public."

Besides the comments from the briefing, the WHO is still compiling an official statement in response to the letter.

Some scientists have also told the New York Times that the WHO is constrained by waiting too long for "enough" evidence to be presented before giving official recommendations, and should instead err on the side of caution. 

But they also note that WHO has a tough job, playing diplomat at the same time as assessing scientific research.

Scientific controversy

This isn't the first time the risk of airborne transmission has been debated in the scientific community.

A paper from Nobel Laureate Mario Molino sparked the ire of more than 40 scientists in a paper that purported that the coronavirus's main route of infection was through airborne transmissions. They penned a letter to the journal to retract the study, citing flaws in methodology, lack of expertise and a dubious review process. 

The paper also criticised the WHO's narrow focus on contact transmission.

Molina and other authors penned a rebuttal against the letter, saying that certain sentences were taken out of context and "it is truly incredible how the authors could come up with such naive ideas, merely because no Covid-19 epidemiologists were among the authors of our paper".

So far, the study has not been retracted. 

READ: Scientists are calling for retraction of Covid-19 paper by Nobel laureate

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